Flat panel electronic displays are currently finding increased application due to the reduced depth requirement of the display as well as low power requirements in the case of large area displays. In particular backlighted color liquid crystal displays have shown a superior color gamut over the conventional color cathode ray tube display. Color is achieved in these LCD displays through use of a fluorescent lamp illumination on the back of the display with local control of transmitted light from the rear illuminator provided by triad groupings of red, green and blue pixel control elements. The fluorescent lamp contains an emission spectra showing intensity peaks for the red, green and blue components required to provide a full color gamut. The pixel control elements are typically composed of a twisted nematic liquid crystal with internal color filter at each pixel. The pixels are arranged in a row and column matrix which can be addressed electrically by row and column address electrodes so that each cell can be individually controlled in transmission to provide a required amount of colored light at that pixel. Matrix addressing of all the pixels in the display can provide a high fidelity color image exhibiting a continuous range in color as well as intensity.
The recent introduction of the notebook computer capitalizes on the high information density, high speed and low power requirements achieved through electronic microminiaturization. Notebook computers using monochrome LCD displays have received wide acceptance. A major problem in providing color displays to these computers is the power requirement of the backlight system as well as the cost of high contrast color LCD displays. The present invention addresses the illumination inefficiencies of existing color LCD's as well as the complexity and cost of the color LCD display.
Present notebook color LCD displays utilize a lightpipe type of backlight illumination system. The lightpipe is edgelighted along one or two edges of the plastic lightpipe with high efficiency fluorescent lamps. The lamps are small in diameter and contain a phosphor with peak intensities at the required red, green and blue color wavelength to produce a full color gamut display. The surface of the lightpipe contains optical discontinuities causing scattering of the internally transmitted light so that light escapes from the lightpipe at the scattering sites. The size of the scattering sites is adjusted as a function of the distance from the fluorescent lamp so that an even distribution of escaping light results from the face of the lightpipe. Typically the LCD incorporates the red-green-blue (RGB) filter array within the device. White light from the adjacent lightpipe is filtered to provide the appropriate color transmission for that pixel. At least 66% of the light is adsorbed by the filter since only one component of the three is transmitted. In practice less than 7% of the input light from the backlight is finally transmitted to the observer. Greater than 80% of the battery power on a color LCD notebook computer is spent in the backlight system.
Furthermore the diffuse light from the lightpipe illuminates the LCD pixel from many different incident angles. Most efficient modulation and best display contrast occurs for normally incident light rays so that a collimated backlight is desired. In the more simple (lower cost) LCD device using a super twist nematic (STN) display, contrast is inversely proportional to the duty cycle of addressing which is proportional to the number of matrix addressed lines in the display. The more expensive thin film transistor LCD (TFT-LCD) is used in order to provide a memory function at each pixel to overcome the duty cycle limitation and which results in display high contrast. Therefore a collimated backlight will allow use of the lower cost STN-LCD in place of the TFT-LCD for equivalent display contrast. For these reasons there is a need for a thin high efficiency backlight illuminator that eliminates the loss due to the color filters and also allows use of the lower cost STN-LCD technology.
Prior art describes rear illumination by means of a scattering box behind RGB interference filters located over slit apertures in register with RGB pixel elements of an adjacent LCD. Transmitted light through each filter is collimated by a cylindrical lens array onto the adjacent LCD (French, Stewart--U.S. Pat. No. 4,924,356 (1990). While the interference filter is more energy efficient than an absorptive filter in that light energy outside the passband of the filter is reflected back into the scattering box, the efficiency of the scattering box is substantially less than a light pipe using total internal reflection as a propagation means for light distribution. In addition light collection efficiency for the cylindrical lens array for the scattered light input from the scattering box is at best 30%. Finally confining collimation Of the transmitted light from each aperture to pixel rows within the LCD in order to maintain color purity is very difficult by the proposed geometry. For instance a conventional VGA notebook LCD display contains 0.12 mm wide pixel rows under a 0.7 mm thick rear glass of the LCD so that a maximum diverge angle of .+-.5.6 degrees (tan.sup.- 1 (0.06/0.7) results in order to preserve color purity. Likewise with an illumination aperture per pixel row as described in the prior art in which each aperture is 12% of the row width an aperture width of 15.6 microns results which is difficult and costly to fabricate in a large array.
Other prior art describes UV illumination of phosphor stripe patterns external to the LCD without any provision for maintaining color purity in high definition LCD displays as limited by the thickness of the rear glass of the LCD (Canter, Warg, Brooks--U.S. Pat. No. 4,668,049 (1987)). Yet other prior art describes collimation of backlight by means of lenticular lens arrays and subsequent diffraction of light into RGB lines patterns onto the pixel RGB rows of the LCD. Color separation of the white light by this technique is inefficient. The present invention addresses the limitations of the prior art and fulfills the needs for a thin high efficiency large area low cost color backlight for display devices.